(53c) Structure-Function Relationships in Alcohol Dehydration within Lewis Acid Zeotypes

Atom-efficient deoxygenation of oxygenates remains a crucial step in the conversion of biomass-derived molecules to those useful as fuels and platform chemicals. In this work, we examine structure-function relationships for alcohol dehydration within Lewis acid zeotypes of varied group IV metal atom (Ti, Zr, and Hf) within the BEA framework. We describe and contrast reactivity and site requirements for methanol and ethanol over these Lewis acids of well-defined structure. These mechanistic assessments demonstrate how confinement and acid strength influence dehydration rates of reactants that are similar in their electronic interactions with active sites but that differ in size.

Methanol and ethanol dehydrate via bimolecular pathways to form alkyl ethers via pathways requiring adsorption of two alcohol molecules to tetrahedrally coordinated Lewis acid sites, followed by subsequent dehydration (via nucleophilic substitution) to form a product dialkyl ether and a dissociated water that condenses to desorb water and complete the catalytic cycle. Transition states for alcohols of higher carbon number have more effective contacts with the surrounding zeotype matrix as a result of their size and shape, compared to methanol. Dimer-like alcohol precursors, however, are similar in their size and shape to their respective transition states. As a result, confining voids preferentially stabilize transition states when taken with respect to an adsorbed alcohol precursor and a gas phase alcohol but not when active sites are covered by alcohol dimers. Differences in reactivity among zeotype materials of varied group IV metal atom reflect the differences in charge at transition states compared to at neutral adsorbed precursors and how they are stabilized by Lewis acid-base interactions.